.back{
    background-color: yellow;
    background: url("../images/3.png");
    background-size: cover;
    background-repeat: no-repeat;
}
.invite_btn{
    border: none;
    background-color: transparent;
    background-size: cover;
    margin-left: 25%;
    margin-top: 80%;
}
.pre{
    background-image: url("../images/invite1.png");
}
.pre:active{
    background-image: url("../images/invite2.png");
}

.mask{
    display:none;
    position:fixed;
    z-index:9999;
    width:100%;
    height:100%;
    left:0;
    top:0;
    background:#000;
}
.mask img{
    margin-top:5%
}
.mask p{
    color:#ffffff;
    font-size:1.5em
}
.text-center{
    text-align:center
}
.an2{-webkit-animation-name:bounceIn;-webkit-animation-duration:1s;-webkit-animation-timing-function:ease;-webkit-animation-fill-mode:both}